Electrochemical Method Of Producing Single-Layer Or Few-Layer Graphene Sheets

ABSTRACT

A method of producing isolated graphene sheets from a layered graphite, comprising: (a) forming an alkali metal ion-intercalated graphite compound by an electrochemical intercalation which uses a liquid solution of an alkali metal salt dissolved in an organic solvent as both an electrolyte and an intercalate source, layered graphite material as an anode material, and a metal or graphite as a cathode material, and wherein a current is imposed upon a cathode and an anode at a current density for a duration of time sufficient for effecting the electrochemical intercalation of alkali metal ions into interlayer spacing; and (b) exfoliating and separating hexagonal carbon atomic interlayers (graphene planes) from the alkali metal ion-intercalated graphite compound using ultrasonication, thermal shock exposure, exposure to water solution, mechanical shearing treatment, or a combination thereof to produce isolated graphene sheets.

FIELD OF THE INVENTION

The present invention relates to a method of producing thin isolatedgraphene sheets directly from natural graphite mineral or graphite rock.

BACKGROUND

A single-layer graphene sheet is composed of carbon atoms occupying atwo-dimensional hexagonal lattice. Multi-layer graphene is a plateletcomposed of more than one graphene plane. Individual single-layergraphene sheets and multi-layer graphene platelets are hereincollectively called nano graphene platelets (NGPs) or graphenematerials. NGPs include pristine graphene (essentially 99% of carbonatoms), slightly oxidized graphene (<5% by weight of oxygen), grapheneoxide (≦5% by weight of oxygen), slightly fluorinated graphene (<5% byweight of fluorine), graphene fluoride ((≧5% by weight of fluorine),other halogenated graphene, and chemically functionalized graphene.

NGPs have been found to have a range of unusual physical, chemical, andmechanical properties. For instance, graphene was found to exhibit thehighest intrinsic strength and highest thermal conductivity of allexisting materials. Although practical electronic device applicationsfor graphene (e.g., replacing Si as a backbone in a transistor) are notenvisioned to occur within the next 5-10 years, its application as anano filler in a composite material and an electrode material in energystorage devices is imminent. The availability of processable graphenesheets in large quantities is essential to the success in exploitingcomposite, energy, and other applications for graphene.

Our research group was among the first to discover graphene [B. Z. Jangand W. C. Huang, “Nano-scaled Graphene Plates,” U.S. patent applicationSer. No. 10/274,473, submitted on Oct. 21, 2002; now U.S. Pat. No.7,071,258 (Jul. 4, 2006)]. The processes for producing NGPs and NGPnanocomposites were recently reviewed by us [Bor Z. Jang and A Zhamu,“Processing of Nano Graphene Platelets (NGPs) and NGP Nanocomposites: AReview,” J. Materials Sci. 43 (2008) 5092-5101]. Our research hasyielded a process for chemical-free production of isolated nano grapheneplatelets that is novel in that is does not follow the establishedmethods for production of nano graphene platelets outlined below. Inaddition, the process is of enhanced utility in that it is costeffective, and provided novel graphene materials with significantlyreduced environmental impact. Four main prior-art approaches have beenfollowed to produce NGPs. Their advantages and shortcomings are brieflysummarized as follows:

Approach 1: Chemical Formation and Reduction of Graphite Oxide (GO)Platelets

The first approach (FIG. 1) entails treating natural graphite powderwith an intercalant and an oxidant (e.g., concentrated sulfuric acid andnitric acid, respectively) to obtain a graphite intercalation compound(GIC) or, actually, graphite oxide (GO). [William S. Hummers, Jr., etal., Preparation of Graphitic Oxide, Journal of the American ChemicalSociety, 1958, p. 1339.] Prior to intercalation or oxidation, graphitehas an inter-graphene plane spacing of approximately 0.335 nm(L_(d)=½d₀₀₂=0.335 nm). With an intercalation and oxidation treatment,the inter-graphene spacing is increased to a value typically greaterthan 0.6 nm. This is the first expansion stage experienced by thegraphite material during this chemical route. The obtained GIC or GO isthen subjected to further expansion (often referred to as exfoliation)using either a thermal shock exposure or a solution-based,ultrasonication-assisted graphene layer exfoliation approach.

In the thermal shock exposure approach, the GIC or GO is exposed to ahigh temperature (typically 800-1,050° C.) for a short period of time(typically 15 to 60 seconds) to exfoliate or expand the GIC or GO forthe formation of exfoliated or further expanded graphite, which istypically in the form of a “graphite worm” composed of graphite flakesthat are still interconnected with one another. This thermal shockprocedure can produce some separated graphite flakes or graphene sheets,but normally the majority of graphite flakes remain interconnected.Typically, the exfoliated graphite or graphite worm is then subjected toa flake separation treatment using air milling, mechanical shearing, orultrasonication in water. Hence, approach 1 basically entails threedistinct procedures: first expansion (oxidation or intercalation),further expansion (or “exfoliation”), and separation.

In the solution-based separation approach, the expanded or exfoliated GOpowder is dispersed in water or aqueous alcohol solution, which issubjected to ultrasonication. It is important to note that in theseprocesses, ultrasonification is used after intercalation and oxidationof graphite (i.e., after first expansion) and typically after thermalshock exposure of the resulting GIC or GO (after second expansion).Alternatively, the GO powder dispersed in water is subjected to an ionexchange or lengthy purification procedure in such a manner that therepulsive forces between ions residing in the inter-planar spacesovercome the inter-graphene van der Waals forces, resulting in graphenelayer separations.

There are several major problems associated with this conventionalchemical production process:

-   -   (1) The process requires the use of large quantities of several        undesirable chemicals, such as sulfuric acid, nitric acid, and        potassium permanganate or sodium chlorate.    -   (2) The chemical treatment process requires a long intercalation        and oxidation time, typically 5 hours to five days.    -   (3) Strong acids consume a significant amount of graphite during        this long intercalation or oxidation process by “eating their        way into the graphite” (converting graphite into carbon dioxide,        which is lost in the process). It is not unusual to lose 20-50%        by weight of the graphite material immersed in strong acids and        oxidizers.    -   (4) Both heat- and solution-induced exfoliation approaches        require a very tedious washing and purification step. For        instance, typically 2.5 kg of water is used to wash and recover        1 gram of GIC, producing huge quantities of waste water that        need to be properly treated.    -   (5) In both the heat- and solution-induced exfoliation        approaches, the resulting products are GO platelets that must        undergo a further chemical reduction treatment to reduce the        oxygen content. Typically even after reduction, the electrical        conductivity of GO platelets remains much lower than that of        pristine graphene. Furthermore, the reduction procedure often        involves the utilization of toxic chemicals, such as hydrazine.    -   (6) Furthermore, the quantity of intercalation solution retained        on the flakes after draining may range from 20 to 150 parts of        solution by weight per 100 parts by weight of graphite flakes        (pph) and more typically about 50 to 120 pph.    -   (7) During the high-temperature exfoliation, the residual        intercalate species (e.g. sulfuric acid and nitric acid)        retained by the flakes decompose to produce various species of        sulfuric and nitrous compounds (e.g., NO_(x) and SO_(x)), which        are undesirable. The effluents require expensive remediation        procedures in order not to have an adverse environmental impact.        The present invention was made to overcome the limitations        outlined above.

Approach 2: Direct Formation of Pristine Nano Graphene Platelets

In 2002, our research team succeeded in isolating single-layer andmulti-layer graphene sheets from partially carbonized or graphitizedpolymeric carbons, which were obtained from a polymer or pitch precursor[B. Z Tang and W. C. Huang, “Nano scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/214,473, submitted on Oct. 21, 2002; now U.S.Pat. No. 7,071,258 (Jul. 4, 2006)]. Mack, et al [“Chemical manufactureof nanostructured materials” U.S. Pat. No. 6,872,330 (Mar. 29, 2005)]developed a process that involved intercalating graphite with potassiummetal melt and contacting the resulting K-intercalated graphite withalcohol, producing violently exfoliated graphite containing NGPs. Theprocess must be carefully conducted in a vacuum or an extremely dryglove box environment since pure alkali metals, such as potassium andsodium, are extremely sensitive to moisture and pose an explosiondanger. This process is not amenable to the mass production of NGPs. Thepresent invention was made to overcome the limitations outlined above.

Approach 3: Epitaxial Growth and Chemical Vapor Deposition of NanoGraphene Sheets on Inorganic Crystal Surfaces

Small scale production of ultra-thin graphene sheets on a substrate canbe obtained by thermal decomposition-based epitaxial growth and a laserdesorption-ionization technique. [Walt A. DeHeer, Claire Berger, PhillipN. First, “Patterned thin film graphite devices and method for makingsame” U.S. Pat. No. 7,327,000 B2 (Jun. 12, 2003)] Epitaxial films ofgraphite with only one or a few atomic layers are of technological andscientific significance due to their peculiar characteristics and greatpotential as a device substrate. However, these processes are notsuitable for mass production of isolated graphene sheets for compositematerials and energy storage applications.

Approach 4: The Bottom-Up Approach (Synthesis of Graphene from SmallMolecules)

Yang, et al. [“Two-dimensional Graphene Nano-ribbons,” J. Am. Chem. Soc.130 (2008) 4216-17] synthesized nano graphene sheets with lengths of upto 12 nm using a method that began with Suzuki-Miyaura coupling of1,4-diiodo-2,3,5,6-tetraphenyl-benzene with 4-bromophenylboronic acid.The resulting hexaphenylbenzene derivative was further derivatized andring-fused into small graphene sheets. This is a slow process that thusfar has produced very small graphene sheets.

Hence, an urgent need exists to have a graphene production process thatrequires a reduced amount of undesirable chemical (or elimination ofthese chemicals all together), shortened process time, less energyconsumption, lower degree of graphene oxidation, reduced or eliminatedeffluents of undesirable chemical species into the drainage (e.g.,sulfuric acid) or into the air (e.g., SO₂ and NO₂). The process shouldbe able to produce more pristine (less oxidized and damaged), moreelectrically conductive, and larger/wider graphene sheets.

It may be noted that Kang, et al [F. Kang, “Formic Acid-GraphiteIntercalation Compound,” U.S. Pat. No. 5,698,088 (Dec. 16, 1997)] usedan electrochemical method to intercalate natural flake graphite withformic acid for the purpose of producing flexible graphite products.Zhamu, et al. (U.S. Pat. No. 8,524,067) used an electrochemical methodto intercalate natural graphite with carboxylic acid for the purpose ofproducing graphene sheets. These prior art processes all begin withhighly purified natural graphite, not directly from graphite rock ormined graphite ore.

Natural graphite is obtained by mining graphite-rich ore (commonlyreferred to as graphite rock) and milling it down to the consistency ofsand to allow the graphite to be removed. The milled material is thensubjected to purification treatments that typically include a series offlotation processes to extract the graphite. These procedures are knownto be highly polluting to the environment. Loh, et al. (U.S. Pat. No.9,221,687, issued on Dec. 29, 2015 and US Pub. No. 2013/0102084)discloses an electrochemical method of producing expanded graphite andgraphene sheets from a slurry composed of (a) 15-20 wt. % of graphiterock, (b) 0.1-5 wt. % of graphite flake, and (c)70-80 wt. % of anelectrolyte consisting of 80-160 g/L of LiClO₄ (5-10 wt. %) in propylenecarbonate. Such a process still requires the use of some graphite flakes(purified graphite or expanded graphite) in the reacting slurry.Furthermore, the requirement to have a low proportion (only 15-20 wt. %)of graphite rock and a high electrolyte proportion (70-80 wt. %) in anelectrochemical slurry implies that this is not an economically viableprocess dues to the low production rate and high electrolyte costs.

Clearly, a need exists to have a more cost-effective process thatproduces graphene sheets (particularly single-layer graphene andfew-layer graphene sheets) directly from graphite rock. Such a processnot only avoids the environment-polluting graphite ore purificationprocedures but also makes it possible to have low-cost grapheneavailable. As of today, the graphene, as an industry, has yet to emergemainly due to the extremely high graphene costs that have thus farprohibited graphene-based products from being widely accepted in themarketplace.

SUMMARY OF THE INVENTION

The present invention provides a method of producing isolated graphenesheets having an average thickness smaller than 30 nm (preferably andtypically single-layer graphene or few-layer graphene) directly from alayered graphite material having hexagonal carbon atomic interlayers(graphene planes) with an interlayer spacing (inter-graphene planespacing). The method comprises: (a) forming an alkali metalion-intercalated graphite compound by an electrochemical intercalationprocedure which is conducted in an intercalation reactor, wherein saidreactor contains (i) a liquid solution electrolyte comprising an alkalimetal salt dissolved in an organic solvent; (ii) an anode that containssaid layered graphite material as an active material in ionic contactwith the liquid solution electrolyte; and (iii) a cathode in ioniccontact with the liquid solution electrolyte, and wherein a current isimposed upon a cathode and an anode at a current density for a durationof time sufficient for effecting electrochemical intercalation of alkalimetal ions into the interlayer spacing; and (b) exfoliating andseparating the hexagonal carbon atomic interlayers (isolated graphenesheets) from the alkali metal ion-intercalated graphite compound usingultrasonication alone, thermal shock exposure alone, exposure to watersolution alone, or a combination of thermal shock exposure and amechanical shearing treatment to produce the isolated graphene sheets.

In some embodiments, the organic solvent is selected from 1,3-dioxolane(DOL), 1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether(TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME), diethyleneglycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone,sulfolane, ethylene carbonate (EC), dimethyl carbonate (DMC),methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate,methyl propionate, gamma-butyrolactone (γ-BL), acetonitrile (AN), ethylacetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene,methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate(VC), allyl ethyl carbonate (AEC), a hydrofloroether, or a combinationthereof.

Preferably, the layered graphite material is selected from naturalgraphite, synthetic graphite, highly oriented pyrolytic graphite,graphite fiber, graphitic nano-fiber, graphite rock or graphite mineral,or a combination thereof.

Preferably, the alkali metal salt is selected from sodium perchlorate(NaClO₄), potassium perchlorate (KClO₄), sodium hexafluorophosphate(NaPF₆), potassium hexafluorophosphate (KPF₆), sodium borofluoride(NaBF₄), potassium borofluoride (KBF₄), sodium hexafluoroarsenide,potassium hexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃),potassium trifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethylsulfonylimide sodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide(NaTFSI), bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), asodium ionic liquid salt, lithium perchlorate (LiClO₄), lithiumhexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithiumhexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate (LiCF₃SO₃),bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithiumbis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄),lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-Fluoroalkyl-Phosphates (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethysulfonylimide (LiBETI), lithiumbis(trifluorornethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid lithiumsalt, or a combination thereof.

In some embodiments, the layered graphite material contains multiplegraphite particles dispersed in the liquid solution electrolyte anddisposed in an anode compartment, which multiple graphite particles aresupported or confined by an anode current collector in electroniccontact with the layered graphite material and wherein the anodecompartment and said multiple graphite particles supported thereon orconfined therein are not in electronic contact with said cathode. Themultiple graphite particles may be simply confined in a porous cage atthe anode side. The porous cage is immersed in the liquid solutionelectrolyte so that these multiple graphite particles remain essentiallydispersed in the electrolyte. Preferably, the multiple graphiteparticles are clustered together to form a network ofelectron-conducting pathways, electronically connected to the currentcollector. The current collector is made of an electrically conductivematerial (e.g. Cu foil, Cu foam, graphite foam, Ni foam, etc.).

In some embodiments, the invention provides a method of producingisolated graphene sheets having an average thickness smaller than 30 nm(typically <10 nm and, mostly, single-layer or few-layer graphene)directly from a layered graphite material having hexagonal carbon atomicinterlayers (graphene planes) with an interlayer spacing (inter-grapheneplane space). The method comprises:

-   (a) forming an alkali metal ion-intercalated graphite compound by an    electrochemical intercalation procedure which is conducted in an    intercalation reactor (e.g. a chamber, a tank, or other container),    wherein said reactor contains (i) a liquid solution electrolyte    comprising an alkali metal salt dissolved in a liquid organic    solvent; (ii) an anode that contains said layered graphite material    as an active material in ionic contact with the liquid solution    electrolyte; and (iii) a cathode in ionic contact with the liquid    solution electrolyte, and wherein a current is imposed upon a    cathode and an anode at a current density for a duration of time    sufficient for effecting electrochemical intercalation of alkali    metal ions into the interlayer spacing, wherein the alkali metal    salt is selected from sodium perchlorate (NaClO₄), potassium    perchlorate (KClO₄), sodium hexafluorophosphate (NaPF₆), potassium    hexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassium    borofluoride (KBF₄), sodium hexafluoroarsenide, potassium    hexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃),    potassium trifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl    sulfonylimide sodium (NaN(CF₃SO₂)₂), sodium    trifluoromethanesulfonimide (NaTFSI), bis-trifluoromethyl    sulfonylimide potassium (KN(CF₃SO₂)₂), a sodium ionic liquid salt,    lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄),    lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate    (LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium    (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithium    oxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate    (LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-Fluoroalkyl-Phosphates    (LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethysulfonylimidc (LiBETI),    lithium bis(trifluoromethanesulphonyl)imide, lithium    bis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide    (LiTFSI), an ionic liquid lithium salt, or a combination thereof;    and-   (b) exfoliating and separating the hexagonal carbon atomic    interlayers (graphene sheets) from the alkali metal ion-intercalated    graphite compound using ultrasonication alone, thermal shock    exposure alone, exposure to water solution alone, or a combination    of thermal shock exposure and a mechanical shearing treatment to    produce isolated graphene sheets. The mechanical shearing treatment    may comprise using air milling, air jet milling, ball milling,    rotating-blade mechanical shearing, ultrasonication, or a    combination thereof. In a preferred embodiment, thermal shock    exposure comprises heating said intercalated graphite to a    temperature in the range of 300-1,200° C. for a period of 15 seconds    to 2 minutes.

In some preferred embodiments, the organic solvent is selected flow1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycoldimethylether (TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME),diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE),sulfone, sulfolane, ethylene carbonate (EC), propylene carbonate (PC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), ethyl propionate, methyl propionate, gamma-butyrolactone (γ-BL),acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methylformate (MF), toluene, xylene, methyl acetate (MA), fluoroethylenecarbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC), ahydrofloroether, or a combination thereof.

Again, preferably, the layered graphite material is selected fromnatural graphite, synthetic graphite, highly oriented pyioltic graphite,graphite fiber, graphitic nano-fiber, graphite rock, or a combinationthereof. In a desirable embodiment, the layered graphite materialcontains graphite rock or mined graphite ore only, and no other graphitematerial is included in the anode or dispersed in the liquid solution(the electrolyte).

The imposing current used in the electrochemical reaction preferablyprovides a current density in the range of 1 to 600 A/m², morepreferably in the range of 20 to 400 A/m², and most preferably in therange of 100 to 300 A/m². The exfoliation step preferably comprisesheating the intercalated graphite to a temperature in the range of300-1,250° C. for a duration of 10 seconds to 2 minutes, most preferablyat a temperature in the range of 400-1,000° C. for a duration of 30-60seconds.

Preferably, the alkali metal ion-intercalated graphite compound containsStage-1 and/or Stage-2 graphite intercalation compounds. Stage-1graphite intercalation compounds are preferred. A Stage-n graphiteintercalation compound is a graphite material wherein there is one layerof intercalate species (or intercalant) for each n graphene layers.

Typically, the produced graphene sheets contain mostly single-layergraphene and/or few-layer graphene having 2-10 hexagonal carbon atomicinterlayers or graphene planes.

In a typical operation, the method induces intercalation of lithium,sodium, and/or potassium ions (all positive ions, Li⁺, Na⁺, and K⁺).However, if so desired, the electrochemical potential at the anode andthat at the cathode and the current density may be adjusted in such amanner that negative ions (anions), the solvent, or solvated ions can beintercalated into inter-graphene spaces as well.

The method may comprise additional steps of electrochemicalintercalation of the multi-layer graphene sheets to obtain a furtherintercalated compound (intercalated multi-layer graphene sheets), andexfoliation of the further intercalated compound to produce thinnergraphene sheets (mostly single-layer), with or without a subsequentmechanical shearing treatment. Specifically, the method may furthercomprise a step of re-intercalating the isolated graphene sheets usingan electrochemical or chemical intercalation method to obtainintercalated graphene sheets and a step of exfoliating and separatingthe intercalated graphene sheets to produce substantially allsingle-layer graphene sheets using ultrasonication, thermal shockexposure, exposure to water solution, mechanical shearing treatment, ora combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A flow chart showing an embodiment of the presently inventedmethod of producing isolated graphene sheets.

FIG. 2 Schematic drawing of an apparatus for electrochemicalintercalation of graphite.

FIG. 3(A) SEM image of graphene sheets produced by the presentlyinvented electrochemical intercalation method.

FIG. 3(B) Transmission electron micrograph of graphene sheets producedby conventional chemical intercalation and oxidation of graphite usingstrong sulfuric acid and highly oxidizing agent.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Carbon materials can assume an essentially amorphous structure (glassycarbon), a highly organized crystal (graphite), or a whole range ofintermediate structures that are characterized in that variousproportions and sizes of graphite crystallites and defects are dispersedin an amorphous matrix. Typically, a graphite crystallite is composed ofa number of graphene sheets or basal planes that are bonded togetherthrough van der Waals forces in the c-axis direction, the directionperpendicular to the basal plane. These graphite crystallites aretypically micron- or nanometer-sized. The graphite crystallites aredispersed in or connected by crystal defects or an amorphous phase in agraphite particle, which can be a graphite flake, carbon/graphite fibersegment, carbon/graphite whisker, or carbon/graphite nano-fiber. In thecase of a carbon or graphite fiber segment, the graphene plates may be apart of a characteristic “turbostratic structure.” Basically, a graphitematerial is composed of many graphene planes (hexagonal carbon atomicinterlayers) stacked together having inter-planar spacing. Thesegraphene planes can be exfoliated and separated to obtain isolatedgraphene sheets that can each contain one graphene plane or severalgraphene planes of hexagonal carbon atoms.

One preferred specific embodiment of the present invention is a methodof producing isolated graphene sheets, also called nano-scaled grapheneplatelets (NGPs), from a graphitic material (preferably directly fromgraphite rock or graphite ore without purification). Each grapheneplane, also referred to as a graphene sheet or basal plane, comprises atwo-dimensional hexagonal structure of carbon atoms. Each graphene sheethas a length and a width parallel to the graphene plane and a thicknessorthogonal to the graphene plane. By definition, the thickness of an NGPis 100 nanometers (nm) or smaller (more typically <10 nm and mosttypically and desirably <3.4 nm), with a single-sheet NGP (single-layergraphene) being as thin as 0.34 nm. The length and width of a NGP arctypically between 1:m and 20 μm, but could be longer or shorter. Forcertain applications, both length and width are smaller than 1 μm.

Generally speaking, as schematically shown in FIG. 1, a method has beendeveloped for converting a layered or laminar graphite material 10 toisolated graphene sheets 16 having an average thickness smaller than 30nm, more typically smaller than 10 nm, and further more typicallythinner than 3.4 nm (in many cases, mostly single-layer graphene). Themethod comprises (a) forming an alkali metal ion-intercalated graphitecompound 12 by an electrochemical intercalation procedure, which uses aliquid solution (composed of an alkali metal salt dissolved in anorganic solvent) as both an electrolyte and an intercalate source, thelayered graphite material 10 as an anode material, and a metal orgraphite (preferably alkali metal impregnated metal or graphite foam) asa cathode material, and wherein a current is imposed upon the cathodeand the anode at a current density for a duration of time sufficient foreffecting the electrochemical intercalation; and (b) exposing the alkalimetal-intercalated graphite compound 12 to a thermal shock, a watersolution exposure, and/or an ultrasonication (or other mechanicalshearing) treatment.

In this Step (b), thermal shock exposure may be conducted if someorganic species have been intercalated into inter-graphene plane spacesto produce separated graphene sheets. If the anode contains Stage-Igraphite intercalation compounds, thermal shock alone can produceseparated graphene sheets 16. Otherwise, thermal shock leads to theformation of exfoliated graphite 14 (also referred to as graphiteworms), which is then subjected a mechanical shearing treatment orultrasonication to produce the desired isolated graphene sheets 16. Ifthe graphite intercalation compounds contain mainly alkali metal ions(Li, Na, and/or K) residing in inter-graphene plane spaces, theresulting alkali metal-intercalated graphite compounds may be immersedin water or water-alcohol solution (with or without sonication) toeffect exfoliation and separation of graphene sheets.

The exfoliation step preferably comprises heating the intercalatedgraphite to a temperature in the range of 300-1,200° C. for a durationof 10 seconds to 2 minutes, most preferably at a temperature in therange of 600-1,000° C. for a duration of 30-60 seconds. The exfoliationstep in the instant invention does not involve the evolution ofundesirable species, such as NO_(x) and SO_(x), which are commonby-products of exfoliating conventional sulfuric or nitricacid-intercalated graphite compounds.

Schematically shown in FIG. 2 is an apparatus that can be used forelectrochemical intercalation of graphite according to a preferredembodiment of the present invention. The apparatus comprises a container32 to accommodate electrodes and electrolyte. The anode is comprised ofmultiple graphite particles 40 that are dispersed in a liquid solutionelectrolyte (e.g., a NaClO₄ +propylene carbonate, also an intercalatesource) and are supported by a porous anode supporting element 34,preferably a porous metal plate, such as nickel, titanium, or stainlesssteel. The graphite particles 40 preferably form a continuous network ofelectron-conducting pathways with respect to the anode support plate 34,but are accessible to the intercalate. In some preferred embodiments,such a network of electron-conducting pathways may be achieved bydispersing and packing >20% by wt. of graphite rock or graphite ore(preferably >30% by wt. and more preferably >40% by wt.), plus someoptional conductive fillers, in the electrolyte. An electricallyinsulating, porous separator plate 38 (e.g., Teflon fabric or glassfiber mat) is placed between the anode and the cathode 36 (e.g., aporous graphite impregnated with Li, Na, and/or K) or a Li/Na/K metalplate) to prevent internal short-circuiting. A DC current source 46 isused to provide a current to the anode support element 34 and thecathode 36. The imposing current used in the electrochemical reactionpreferably provides a current density in the range of 50 to 600 A/m²,most preferably in the range of 100 to 400 A/m². Fresh electrolyte(intercalate) may be supplied from an electrolyte source (not shown)through a pipe 48 and a control valve 50. Excess electrolyte may bedrained through a valve 52. In some embodiments, the electrolyte cancontain the graphite material to be intercalated dispersed therein andan additional amount of this electrolyte (appearing like a slurry) maybe continuously or intermittently introduced into the intercalationchamber. This will make a continuous process.

Thus, in some embodiments, the invention provides a method of producingisolated graphene sheets having an average thickness smaller than 10 nmdirectly from a layered graphite material having hexagonal carbon atomicinterlayers with an interlayer spacing, the method comprising:

-   (a) forming an alkali metal ion-intercalated graphite compound in an    electrochemical intercalation reactor, wherein the reactor contains    at least an anode, a cathode, and a liquid solution electrolyte in    physical contact with both the anode and the cathode, wherein the    liquid solution electrolyte is composed of an alkali metal salt    dissolved in an organic solvent and the layered graphite material    (to be intercalated) contains milled graphite rock or graphite    mineral dispersed in the liquid solution electrolyte at a    concentration higher than 20% by weight (preferably higher than 30%,    further preferably >40%, even more preferably higher than 50%, and    most preferably >60%), and wherein a current is imposed upon the    cathode and the anode at a current density for a duration of time    sufficient to enable electrochemical intercalation of alkali metal    ions into the interlayer spacing; and-   (b) exfoliating and separating the hexagonal carbon atomic    interlayers from the alkali metal ion-intercalated graphite compound    using ultrasonication, thermal shock exposure, exposure to water    solution, mechanical shearing treatment, or a combination thereof to    produce isolated graphene sheets.    Preferably, the concentration of the milled graphite rock or    graphite mineral in the liquid solution electrolyte is sufficiently    high to achieve a network of electron-conducting pathways, which are    in electronic contact with an anode (e.g. via an anode current    collector), but not with a cathode.

In an alternative electrochemical intercalation configuration, all thegraphite materials to be intercalated and then exfoliated (e.g. graphiterook, natural graphite, artificial graphite, graphite fibers, etc.) maybe formed into a rod or plate that serves as an anode electrode. Analkali metal or alkali metal-containing rod or plate serves as acathode, and a liquid solution containing alkali metal salt dissolved inan organic solvent serves as the electrolyte. In this alternativeconfiguration, no graphite material to be intercalated (e.g. no graphiterock, graphite mineral or mined graphite ore) is dispersed in the liquidelectrolyte. A current is then imposed to the anode and the cathode toallow for electrochemical intercalation of alkali metal ions (Li⁺, Na⁺,and/or K⁺) into the anode active material (the graphite material to beexfoliated) to produce the alkali metal-intercalated graphite, which isthen exfoliated by using the procedures described in step (b).

The layered graphite material to be intercalated in all theaforementioned electrochemical intercalation configurations may beselected from milled graphite rock (unpurified graphite ore or graphitemineral), natural graphite (purified), synthetic graphite, highlyoriented pyrolytic graphite, graphite fiber, graphitic nano-fiber,graphite oxide, graphite fluoride, chemically modified graphite,expandable graphite, expanded graphite, or a combination thereof

The mechanical shearing treatment, used to further separate graphiteflakes and possibly reduce the flake size, preferably comprises usingair milling (including air jet milling), ball milling, mechanicalshearing (including rotating blade fluid grinding), any fluid energybased size-reduction process, ultrasonication, or a combination thereof.

Preferably, the alkali metal salt is selected from sodium perchlorate(NaClO₄), potassium perchlorate (KClO₄), sodium hexafluorophosphate(NaPF₆), potassium hexafluorophosphate (KPF₆), sodium borofluoride(NaBF₄), potassium borofluoride (KBF₄), sodium hexafluoroarsenide,potassium hexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃),potassium trifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethylsulfonylimide sodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide(NaTFSI), bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), asodium ionic liquid salt, lithium perchlorate (LiClO₄), lithiumhexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithiumhexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate (LiCF₃SO₃),bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithiumbis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄),lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-Fluoroalkyl-Phosphates (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethysulfonylimide (LiBETI), lithiumbis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide,lithium trifluoromethanesulfonimide (LiTFST), an ionic liquid lithiumsalt, or a combination thereof.

Preferably, the organic solvent is selected from 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, propylenecarbonate, ethylene carbonate (EC), dimethyl carbonate (DMC),methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate,methyl propionate, gamma-butyrolactone (γ-BL), acetonitrile (AN), ethylacetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene,methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate(VC), allyl ethyl carbonate (AEC), a hydrofloroether, or a combinationthereof. It is quite unexpected that essentially all of these solventscan be used in the present electrochemical intercalation method.

It is quite surprising that sodium ions and potassium ions, albeitsignificantly larger than lithium ions in terms of ionic radii, can beintercalated into all kinds of graphite materials using the instantelectrochemical configurations and method. Further unexpectedly, mixedions (e.g. Li⁺+Na⁺, or Li⁺+K⁺) intercalated into inter-graphene planespacing of a graphite material are more effective than single-ionspecies (e.g. Li⁺ only) in exfoliating graphite to form thinner graphenesheets.

We have found that the invented electrochemical intercalation (withcertain alkali metal salts and certain solvents) and thermal exfoliationcan led to the formation of NGPs with an average thickness smaller than5 nm. However, stage-2 and stage-3 graphite intercalation compounds canlead to graphene platelets thicker than 5 nm. In order to further reducethe platelet thickness, we have conducted further studies and found thatrepeated electrochemical intercalations/exfoliations are an effectivemethod of producing ultra-thin, nano-scaled graphene platelets with anaverage thickness smaller than 2 nm or 5 graphene sheets and, in manycases, mostly single-layer graphene.

It may be noted that, in a traditional graphite intercalation compound(GIC) obtained by intercalation of a laminar graphite material, theintercalant species may form a complete or partial layer in aninter-layer space or gallery. If there always exists one graphene layerbetween two neighboring intercalant layers, the resulting graphite isreferred to as a Stage-1 GIC (i.e. on average, there is oneintercalation layer per one graphene plane). If n graphene layers existbetween two intercalant layers, we have a Stage-n GIC. Alkalimetal-intercalated graphite compounds were found to be stage-2, stage-3,stage-4, or stage-5, depending on the type of carboxylic acid used. Itis generally believed that a necessary condition for the formation ofall single-layer graphene is to have a perfect Stage-1 GIC forexfoliation. Even with a Stage-1 GIC, not all of the graphene layers getexfoliated for reasons that remain unclear. Similarly, exfoliation of aStage-n GIC (with n>5) tends to lead to a wide distribution of NGPthicknesses (mostly much greater than n layers). In other words,exfoliation of Stage-5 GICs often yields NGPs much thicker than 10 or 20layers. Hence, a major challenge is to be able to consistently produceNGPs with well-controlled dimensions (preferably ultra-thin) fromacid-intercalated graphite. In this context, it was surprising for us todiscover that the instant method can consistently lead to the formationof few-layer graphene and/or single-layer graphene only. The productionyield is typically higher than 70%, more typically higher than 80%, andmost typically higher than 90%.

The following examples serve to provide the best modes of practice forthe present invention and should not be construed as limiting the scopeof the invention:

EXAMPLE 1 Production of Isolated Graphene Sheets from Synthetic Graphite

One gram of meso-carbon micro-beads (MCMBs), having an average diameter<15 μm, were used as the anode material and 1,000 mL of a liquidsolution electrolyte (typically 0.5-3 M of an alkali metal salt in anorganic solvent). Ethylene carbonate (EC), propylene carbonate (PC), anddiethyl carbonate (DEC) were used as the solvent. The alkali metal saltsused in this example include lithium perchlorate (LiClO₄), sodiumperchlorate (NaClO₄), potassium perchlorate (KClO₄), and their mixtures.

The anode supporting element is a stainless steel plate and the cathodeis a graphite foam of approximately 4 cm in diameter and 0.2 cm inthickness, impregnated with lithium or sodium. The separator, a glassfiber fabric, was used to separate the cathode plate from the MCMBparticles and to compress these particles down against the anodesupporting element to ensure that the MCMBs are in electrical connectionwith the anode supporting element to serve as the anode. The electrodes,electrolyte, and separator are contained in a Buchner-type funnel toform an electrochemical cell. The anode supporting element, the cathode,and the separator are porous to permit intercalate (contained in theelectrolyte) to saturate the graphite and to pass through the cell fromtop to bottom.

The MCMBs were subjected to an electrochemical charging treatment (i.e.charging alkali metal ions into inter-graphene plane spaces in a MCMB ata current of 0.5 amps (current density of about 0.04 amps/cm²) and at acell voltage of about 4-6 volts for 2-5 hours. These values may bevaried with changes in cell configuration and makeup. Followingelectrochemical charging treatment, the resulting intercalated particles(beads) were washed with water and dried.

Subsequently, some of the alkali metal ion-intercalated compound wastransferred to a water bath. The compound, upon contact with water, wasfound to induce extremely rapid and high expansions of graphitecrystallites. Subsequently, some portion of this expanded/exfoliatedgraphite solution was subjected to sonication. Various samples werecollected with their morphology studied by SEM and TEM observations andtheir specific surface areas measured by the well-known BET method.

TABLE 1 Results of varying types of alkali metal salts and solvents.Starting Specific surface Sample graphite Alkali metal salt Solvent area(m²/g) Comments M-1 MCMBs LiClO₄ EC 720 >65% single-layer M-2 MCMBsNaClO₄ EC 810 >80% single-layer M-3 MCMBs KClO₄ EC 625 >40% single-layerM-4 MCMBs LiClO₄ + NaClO₄ EC 885 >85% single-layer M-5 MCMBs LiClO₄ +KClO₄ EC 730 >70% single-layer M-6 MCMBs NaClO₄ PC 683 >60% single-layerM-7 MCMBs LiClO₄ PC 640 >50% single-layer

Several important observations may be made from the data in this table:

-   -   1) Larger alkali metal ions (Na⁺ and K⁺), relative to Li⁺, are        also effective intercalant when it comes to the production of        ultra-thin graphene sheets. Actually, Na⁺ ions are more        effective than Li⁺ in this aspect.    -   2) A mixture of two alkali metal salts (e.g. LiClO₄+NaClO₄) is        more effective than single components alone in producing        single-layer graphene sheets.    -   3) EC appears to be more effective than PC.    -   4) Products containing a majority of graphene sheets being        single-layer graphene can he readily produced using the        presently invented electrochemical intercalation method.        Approximately one half of these NGPs were then subjected to        re-intercalation under comparable electrochemical intercalation        conditions to obtain re-intercalated NGPs. Subsequently, these        re-intercalated NGPs were transferred to an ultrasonication bath        ultra-thin NGPs. Electron microscopic examinations of selected        samples indicate that the majority of the resulting NGPs are        single-layer graphene sheets.

COMPARATIVE EXAMPLE 1 Sulfuric-Nitric Acid-Intercalated MCMBs

One gram of MCMB beads as used in Example 1 were intercalated with amixture of sulfuric acid, nitric acid, and potassium permanganate at aweight ratio of 4:1:0.05 (graphite-to-intercalate ratio of 1:3) for fourhours. Upon completion of the intercalation reaction, the mixture waspoured into deionized water and filtered. The sample was then washedwith 5% HCl solution to remove most of the sulfate ions and residualsalt and then repeatedly rinsed with deionized water until the pH of thefiltrate was approximately 5. The dried sample was then exfoliated at1,000° C. for 45 seconds. The resulting NGPs were examined using SEM andTEM and their length (largest lateral dimension) and thickness weremeasured. It was observed that the presently invented electrochemicalintercalation method leads to graphene sheets of comparable thicknessdistribution, but much larger lateral dimensions (3-5 μm vs. 200-300nm). Graphene sheets were made into graphene paper layer using awell-known vacuum-assisted filtration procedure. The graphene paperprepared from hydrazine-reduced graphene oxide (made fromsulfuric-nitric acid-intercalated MCMBs) exhibits electricalconductivity values of 25-350 S/cm. The graphene paper prepared from therelatively oxidation-free graphene sheets made by the presently inventedelectrochemical intercalation exhibit conductivity values of 2,500-4,500S/cm.

EXAMPLE 2 Graphene Sheets from Milled Graphite Rock (Graphite Ore orMineral)

Samples of two grams each of graphite rock containing 56% natural flakegraphite were milled down to 50 mesh particle size. The powder sampleswere subjected to similar electrochemical intercalation conditionsdescribed in Example 1, but with different alkali metal salts andsolvents. The graphite rock powder samples were subjected to anelectrochemical intercalation treatment at a current of 0.5 amps(current density of about 0.04 amps/cm²) and at a cell voltage of about6 volts for 3 hours. Following the electrochemical intercalationtreatment, the resulting intercalated flake was removed from theelectrochemical reactor and dried.

Subsequently, the intercalated compound was transferred to a furnacepre-set at a temperature of 950° C. for 45 seconds. The compound wasfound to induce rapid and high expansions of graphite crystallites withan expansion ratio of greater than 100. After a mechanical shearingtreatment in a high-shear rotating blade device for 15 minutes, theresulting NGPs exhibit a Thickness ranging from single-layer graphenesheets to 8-layer graphene sheets based on SEM and TEM observations.Results are summarized in Table 2 below:

TABLE 2 Results of varying types of alkali metal salts and solvents.Starting Alkali metal Specific surface Sample graphite salt Solvent area(m²/g) Comments R-1 Graphite rock LiPF₆ PC 712 >65% single-layer R-2Graphite rock LiPF₆ + NaPF₆ PC 780 >75% single-layer R-3 Graphite rockLiBF₄ PC 670 >60% single-layer R-4 Graphite rock LiTFSI PC + EC 675 >60%single-layer R-5 Graphite rock LiPF₆ DOL 630 >50% single-layer R-6Graphite rock LiPF₆ DME 665 >60% single-layer R-7 Graphite rock LiPF₆PC + DEC 640 >50% single-layer

It may be noted that the interstitial spaces between two hexagonalcarbon atomic planes (graphene planes) are only approximately 0.28 nm(the plane-to-plane distance is 0.34 nm). A skilled person in the artwould predict that larger molecules and/or ions (K⁺ vs. Li⁺) cannotintercalate into interstitial spaces of a layered graphite material.After intensive R&D efforts we found that electrochemical methods with aproper combination of an alkali metal salt and solvent, and an adequatemagnitude of the imposing current density could be used to open up theinterstitial spaces to accommodate much larger molecules and/or ions.

Re-intercalation of these NGPs and subsequent exfoliation resulted infurther reduction in platelet thickness, with an average thickness ofapproximately 0.75 nm (approximately 2 graphene planes on average).

EXAMPLE 3 Production of Isolated Graphene Sheets from ElectrochemicalInteraction, Exfoliation, and Separation of Purified Natural Graphite

Samples of two grams each of purified natural graphite powder weremilled down to 50 mesh particle size. The powder samples were subjectedto similar electrochemical intercalation conditions described in Example1, but with different alkali metal salts and solvents. The naturalgraphite samples were subjected to an electrochemical intercalationtreatment at a current of 0.5 amps (current density of about 0.04amps/cm²) and at a cell voltage of about 6 volts for 3 hours. Followingthe electrochemical intercalation treatment, the resulting intercalatedgraphite (mostly Stage-1 GIC with some Stage-2) was removed from theelectrochemical reactor and dried.

Subsequently, the intercalated compound was transferred to a furnacepre-set at a temperature of 1,050° C. for 60 seconds. The compound wasfound to induce rapid and high expansions of graphite crystallites withan expansion ratio of greater than 200. After a mechanical shearingtreatment in a high-shear rotating blade device for 15 minutes, theresulting NGPs exhibit a thickness ranging from single-layer graphenesheets to 5-layer graphene sheets based on SEM and TEM observations.Results arc summarized in Table 3 below. It is clear that a wide varietyof alkali metal salts and solvents can be utilized in the presentlyinvented method, making this a versatile and environmentally benignapproach (e.g. as opposed to the conventional method using strongsulfuric acid and oxidizing agents).

TABLE 3 Results of varying types of alkali metal salts and solvents.Starting Specific surface Sample graphite Alkali metal salt Solvent area(m²/g) Comments N-1 Natural LiClO₄ EC + VC 720 >65% single-layergraphite, NG N-2 NG NaClO₄ PC + VC 810 >80% single-layer N-3 NG LiBOB EC635 >45% single-layer N-4 NG LiClO₄ + LiNO₃ EC + PC 824 >80%single-layer N-5 NG LiPF₆ PEGDME 710 >65% single-layer N-6 NG LiPF₆ EC +PC 672 >60% single-layer N-7 NG LiClO₄ PC 630 >45% single-layer

COMPARATIVE EXAMPLE 3 Conventional Hummers Method

Graphite oxide was prepared by oxidation of natural graphite withsulfuric acid, nitrate, and potassium permanganate according to themethod of Hummers [U.S. Pat. No.2,798,878, Jul. 9, 1957]. Uponcompletion of the reaction (10 hours allowed), the mixture was pouredinto deionized water and filtered. The sample was then washed with 5%HCl solution to remove most of the sulfate ions and residual salt andthen repeatedly rinsed with deionized water until the pH of the filtratewas approximately 5. The intent was to remove all sulfuric and nitricacid residue out of graphite interstices. The slurry was spray-dried andstored in a vacuum oven at 65° C. for 24 hours. The interlayer spacingof the resulting powder was determined by the Debey-Scherrer X-raytechnique to be approximately 0.75 nm (7.5 Å), indicating that graphitehas been converted into graphite oxide (Stage-1 and Stage-2 GICs). Thedried, intercalated compound was placed in a quartz tube and insertedinto a horizontal tube furnace pre-set at 950° C. for 35 seconds. Theexfoliated worms were mixed with water and then subjected to amechanical shearing treatment using a high-shear dispersion machine for20 minutes. The resulting graphene sheets were found to have a thicknessof 1-7.2 nm.

EXAMPLE 4 NGPs from Short Carbon Fiber Segments

The procedure was similar to that used in Example 1, but the startingmaterial was graphite fibers chopped into segments with 0.2 mm orsmaller in length prior to electrochemical intercalation by anelectrolyte of NaClO₄+PC/EC. The diameter of carbon fibers wasapproximately 12 μm. After intercalation and exfoliation at 900° C. for30 seconds, the graphene platelets exhibit an average thickness of 6.7nm. Electrochemical re-intercalation of these intermediate-thicknessNGPs with an electrolyte of NaClO₄+PC/EC and subsequent exfoliation ofthe dried re-intercalated compound resulted in the formation ofultra-thin NGPs with an average thickness of 1.1 nm.

EXAMPLE 5 NGPs from Carbon Nano-Fibers (CNFs)

A powder sample of carbon nano-fibers was supplied from Applied Science,Inc. (ASI), Cedarville, Ohio. Approximately 1 gram of CNFs was subjectedto electrochemical intercalations and exfoliations under conditionssimilar to those used in Example 1 (electrolyte of LiPF₆+PC/DEC).Ultra-thin NGPs with an average thickness of 1.2 nm were obtained.

1. A method of producing isolated graphene sheets having an averagethickness smaller than 30 nm directly from a layered graphite materialhaving hexagonal carbon atomic interlayers with an interlayer spacing,said method comprising: (a) forming an alkali metal ion-intercalatedgraphite compound by an electrochemical intercalation procedure which isconducted in an intercalation reactor, wherein said reactor contains (i)a liquid solution electrolyte comprising an alkali metal salt dissolvedin an organic solvent; (ii) an anode that contains said layered graphitematerial as an active material in ionic contact with said liquidsolution electrolyte; and (iii) a cathode in ionic contact with saidliquid solution electrolyte, and wherein a current is imposed upon saidcathode and said anode at a current density for a duration of timesufficient for effecting said electrochemical intercalation of alkalimetal ions into said interlayer spacing, wherein said organic solvent isselected from 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME),tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE),2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylene carbonate (EC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), ethyl propionate, methyl propionate, gamma-butyrolactone (γ-BL),acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methylformate (MF), toluene, xylene, methyl acetate (MA), fluoroethylenecarbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC), ahydrofloroether, or a combination thereof; and (b) exfoliating andseparating said hexagonal carbon atomic interlayers from said alkalimetal ion-intercalated graphite compound using ultrasonication, thermalshock exposure, exposure to water solution, a combination thereof, or acombination thereof with a mechanical shearing treatment to produce saidisolated graphene sheets.
 2. The method of claim 1 wherein said layeredgraphite material is selected from natural graphite, synthetic graphite,highly oriented pyrolytic graphite, graphite fiber, graphiticnano-fiber, graphite rock, or a combination thereof.
 3. The method ofclaim 1 wherein said alkali metal salt is selected from sodiumperchlorate (NaClO₄), potassium perchlorate (KClO₄), sodiumhexafluorophosphate (NaPF₆), potassium hexafluorophosphate (KPF₆),sodium borofluoride (NaBF₄), potassium borofluoride (KBF₄), sodiumhexafluoroarsenide, potassium hexafluoroarsenide, sodiumtrifluoro-metasulfonate (NaCF₃SO₃), potassium trifluoro-metasulfonate(KCF₃SO₃), bis-trifluoromethyl sulfonylimide sodium (NaN(CF₃SO₂)₂),sodium trifluoromethanesulfonimide (NaTFSI), bis-trifluoromethylsulfonylimide potassium (KN(CF₃SO₂)₂), a sodium ionic liquid salt,lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆),lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆),lithium trifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethylsulfonylimide lithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate(LiBOR), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-Fluoroalkyl-Phosphates (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethysulfonylimide (LiBETI), lithiumbis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide,lithium tritluoromethanesulfonimide (LiTFST), an ionic liquid lithiumsalt, or a combination thereof.
 4. A method of producing isolatedgraphene sheets having an average thickness smaller than 30 nm directlyfrom a layered graphite material having hexagonal carbon atomicinterlayers with an interlayer spacing, said method comprising: (a)forming an alkali metal ion-intercalated graphite compound by anelectrochemical intercalation procedure which is conducted in anintercalation reactor, wherein said reactor contains (i) a liquidsolution electrolyte comprising an alkali metal salt dissolved in anorganic solvent; (ii) an anode that contains said layered graphitematerial as an active material in ionic contact with said liquidsolution electrolyte; and (iii) a cathode in ionic contact with saidliquid solution electrolyte, and wherein a current is imposed upon saidcathode and said anode at a current density for a duration of timesufficient for effecting said electrochemical intercalation of alkalimetal ions into said interlayer spacing, wherein said alkali metal saltis selected from sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI),bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), a sodiumionic liquid salt, lithium hexafluorophosphate (LiPF₆), lithiumborofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-Fluoroalkyl-Phosphates(LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethysulfonylimide (LiBETI),lithium bis(trifluoromethanesulphonyl)imide, lithiumbis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI),an ionic liquid lithium salt, or a combination thereof; and (b)exfoliating and separating said hexagonal carbon atomic interlayers fromsaid alkali metal ion-intercalated graphite compound usingultrasonication, thermal shock exposure, exposure to water solution, acombination thereof, or a combination thereof with a mechanical shearingtreatment to produce said isolated graphene sheets.
 5. The method ofclaim 4, wherein said organic solvent is selected from 1,3-dioxolane(DOL), 1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether(TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME), diethyleneglycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone,sulfolane, ethylene carbonate (EC), propylene carbonate (PC), dimethylcarbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC),ethyl propionate, methyl propionate, gamma-butyrolactone (γ-BL),acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methylformate (MF), toluene, xylene, methyl acetate (MA), fluoroethylenecarbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC), ahydrofloroether, or a combination thereof
 6. The method of claim 4,wherein said layered graphite material is selected from naturalgraphite, synthetic graphite, highly oriented pyrolytic graphite,graphite fiber, graphitic nano-fiber, graphite rock, or a combinationthereof.
 7. The method of claim 4, wherein said layered graphitematerial contains graphite rock or mined graphite ore only, and no othergraphite material is included in the anode or dispersed in the liquidsolution.
 8. The method of claim 1, wherein said layered graphitematerial contains graphite rock or mined graphite ore only, and no othergraphite material is included in the anode or dispersed in the liquidsolution.
 9. The method of claim 1, wherein said layered graphitematerial contains graphite rock or mined graphite ore only, and saidgraphite rock or mined graphite is dispersed in the liquid solution at aconcentration higher than 20% by weight.
 10. The method of claim 1,wherein said layered graphite material contains graphite rock or minedgraphite ore only, and said graphite rock or mined graphite is dispersedin the liquid solution at a concentration higher than 20% by weight. 11.A method of producing isolated graphene sheets having an averagethickness smaller than 10 nm directly from a layered graphite materialhaving hexagonal carbon atomic interlayers with an interlayer spacing,said method comprising: (a) forming an alkali metal ion-intercalatedgraphite compound in an electrochemical intercalation reactor, whereinsaid reactor contains at least an anode, a cathode, and a liquidsolution electrolyte in physical contact with both the anode and thecathode, wherein said liquid solution electrolyte is composed of analkali metal salt dissolved in an organic solvent and said layeredgraphite material contains milled graphite rock or graphite mineraldispersed in said liquid solution electrolyte at a concentration higherthan 20% by weight, and wherein a current is imposed upon said cathodeand said anode at a current density for a duration of time sufficientfor effecting electrochemical intercalation of alkali metal ions intosaid interlayer spacing; and (b) exfoliating and separating saidhexagonal carbon atomic interlayers from said alkali metalion-intercalated graphite compound using ultrasonication, thermal shockexposure, exposure to water solution, mechanical shearing treatment, ora combination thereof to produce said isolated graphene sheets.
 12. Themethod of claim 1, wherein said mechanical shearing treatment comprisesusing air milling, air jet milling, ball milling, rotating-blademechanical shearing, or a combination thereof.
 13. The method of claim1, wherein the imposing current provides a current density in the rangeof 1 to 600 A/m².
 14. The method of claim 1, wherein said thermal shockexposure comprises heating said intercalated graphite to a temperaturein the range of 300-1,200° C. for a period of 15 seconds to 2 minutes.15. The method of claim 1, wherein the imposing current provides acurrent density in the range of 20 to 400 A/m².
 16. The method of claim1, wherein said isolated graphene sheets contain single-layer graphene.17. The method of claim 4, wherein said isolated graphene sheets containsingle-layer graphene.
 18. The method of claim 1, wherein said isolatedgraphene sheets contain few-layer graphene having 2-10 hexagonal carbonatomic interlayers or graphene planes.
 19. The method of claim 11,wherein said isolated graphene sheets contain few-layer graphene having2-10 hexagonal carbon atomic interlayers or graphene planes.
 20. Themethod of claim 11, wherein said electrochemical intercalation furtherincludes intercalation of said solvent into the interlayer spacing. 21.The method of claim 1, wherein said alkali metal ion-intercalatedgraphite compound contains Stage-1, Stage-2, or a combination of Stage-1and Stage-2 graphite intercalation compounds.
 22. The method of claim 4,wherein said alkali metal ion-intercalated graphite compound containsStage-1, Stage-2, or a combination of Stage-1 and Stage-2 graphiteintercalation compounds.
 23. The method of claim 1, further comprising astep of re-intercalating said isolated graphene sheets using anelectrochemical or chemical intercalation method to obtain intercalatedgraphene sheets and a step of exfoliating and separating saidintercalated graphene sheets to produce single-layer graphene sheetsusing ultrasonication, thermal shock exposure, exposure to watersolution, mechanical shearing treatment, or a combination thereof. 24.The method of claim 4, further comprising a step of re-intercalatingsaid isolated graphene sheets using an electrochemical or chemicalintercalation method to obtain intercalated graphene sheets and a stepof exfoliating and separating said intercalated graphene sheets toproduce single-layer graphene sheets using ultrasonication, thermalshock exposure, exposure to water solution, mechanical shearingtreatment, or a combination thereof.
 25. The method of claim 11, furthercomprising a step of re-intercalating said isolated graphene sheetsusing an electrochemical or chemical intercalation method to obtainintercalated graphene sheets and a step of exfoliating and separatingsaid intercalated graphene sheets to produce single-layer graphenesheets using ultrasonication, thermal shock exposure, exposure to watersolution, mechanical shearing treatment, or a combination thereof 26.The method of claim 1, wherein said layered graphite material containsmultiple graphite particles dispersed in said liquid solutionelectrolyte and disposed in an anode compartment, which multiplegraphite particles arc supported or confined by an anode currentcollector in electronic contact with said layered graphite material andwherein said anode compartment and said multiple graphite particlessupported thereon or confined therein are not in electronic contact withsaid cathode.
 27. The method of claim 4, wherein said layered graphitematerial contains multiple graphite particles dispersed in said liquidsolution electrolyte and disposed in an anode compartment, whichmultiple graphite particles are supported or confined by an anodecurrent collector in electronic contact with said layered graphitematerial and wherein said anode compartment and said multiple graphiteparticles supported thereon of confined therein are not in electroniccontact with said cathode.
 28. The method of claim 26, wherein saidmultiple graphite particles are clustered together to form a network ofelectron-conducting pathways.
 29. The method of claim 27, wherein saidmultiple graphite particles are clustered together to form a network ofelectron-conducting pathways.